RTX Support for Blender 2.79b
To be able to use RTX cards with 2.79 you can use one of the 'nightly' builds of 2.79 that came between the latest release of 2.79 and 2.8.
From the 'Previous Versions' page (found by clicking the 'Blender 2.7' link from here):

You can also download the experimental nightly build from July 2019
which basically looks like 2.7 plus additional bug fixes and some new
features that later became part of 2.80.

Following that link takes you to here which is where the downloads reside. This allows you to download 2.79.7, which is compatible with RTX cards.
Note: These versions of Blender are the portable versions which run from the folder they are extracted to instead of coming with an installer.